1. Field of the Invention
The present invention relates generally to an oxygen sensor for detecting the oxygen concentration in the exhaust gas of an automobile engine, and more particularly to such an oxygen sensor to be employed in a system capable of simultaneously disposing of the harmful elements in the engine exhaust of an automobile, such as the unburnt hydrocarbons, nitrogen oxides and carbon monoxide, through reaction with a catalyst, and to a method of manufacturing the oxygen sensor.
2. Description of the Prior Art
An oxygen sensor uses a solid electrolyte sintering of an oxygen ion-conductive ceramic material as a partition wall and the oxygen concentration ratio in two spaces divided by this wall is converted to an electromotive force. As is well known, designating the partial pressure of oxygen in one of the spaces as Po.sub.2 (1) and that in the other spaces as Po.sub.2 (2), the electromotive force E generated between the electrodes formed on both sides of the partition wall will be given by: ##EQU1## WHERE R is a gas constant, T is the absolute temperature, and F is the Faraday constant. Thus when Po.sub.2 (1) is known, the unknown Po.sub.2 (2) can be found by measuring the electromotive force E.
In the conventional oxygen sensor, the equilibrium oxygen partial pressure of the atmosphere, or of a mixture of metal and metal oxide, has been used as the oxygen source of known concentration, that is, Po.sub.2 (1). Generally speaking, the following drawbacks have been complained of by users of the conventional oxygen sensor. For instance, in the sensor utilizing the atmosphere as the reference oxygen pole, the mechanism is complicated because an atmosphere inlet tube has to be provided, and invasion of water or mud into such a tube has to be prevented. Further, in the sensor utilizing a mixture of metal and metal oxide as the reference oxygen pole, a long, continuous and stable service of the sensor cannot be expected on account of the metal and metal oxide mixture of Ni/NiO, Cu/CuO, Co/CoO and Fe/FeO, which are known to be available for this purpose, being oxidized or reduced through exposure to the exhaust gas, or of this metal and metal oxide mixture reacting with a solid electrolyte. Furthermore, when such a solid reference oxygen pole is employed as the reference pole, the low temperature performance drops, as for example, at less than 400.degree. C, an electromotive force does not develop, or on account of an increased internal impedance, an apparent drop in the electromotive force occurs, which is a great drawback of this sensor as compared with one utilizing the atmosphere as the reference oxygen pole.